1. Field of the Invention
The invention relates to a transparent layer of polyurethane, as well as to safety glazings covered with such a layer, used in particularly in means to transport and in building, and it deals more particularly with the improvement of the behavior of said glazings when faced with condensation and the formation of fog.
2. Background of the Prior Art
Safety glazings are used more and more in large sizes on transport vehicles or in buildings and in small sizes in eye-glasses, masks, lenses, etc., which safety glazings are of the type comprising a monolithic or laminated support, in glass or plastic, coated through the intermediary of an adhesive layer possibly having energy-absorbing properties, with a transparent plastic layer which is anti-lacerating, that is, should the glazing break, this layer covers the sharp edges of the glass fragments and prevents possible cuts due to said sharp edges; in addition, this layer is auto-cicatrizable, that is, it is composed of a plastic from which localized dents or surface abrasions disappear spontaneously after a short period of time.
Such glazings are described for example in French Patent Publications Nos. 2,187,719, 2,316,913, 2,320,563, 2,398,606 and European Patent Publication Nos. 0,132,198 and 0,131,523. They may, for example, be a laminated glazing comprising two sheets of glass connected by an interposed sheet of plastic, for example polyvinylbutyral, and covered with a sheet of transparent plastic whose surface in contact with the support is essentially thermoplastic, while its external surface is formed essentially of a heat-hardenable polyurethane, or even of a laminated glazing comprising a sheet of glass and a sheet of plastic formed of a polyurethane layer having energy-absorbing properties and an external layer of heat-hardenable polyurethane having auto-cicatrization properties.
The windshields of transport vehicles, in particular, can be subjected to sudden variations in temperature and humidity As soon as their surface temperature goes below the dewpoint of the surrounding air, there is water condensation, which can bother the driver.
Moreover, the effects of the phenomenon vary considerably depending on the type of transparent surface being considered: on bare glass, the drops of water spread much more easily than on an organic film. By simple wiping of bare glass, a relatively regular film of water is formed which is transparent from an optical point of view and whose presence considerably attenuates the effects of condensation. In general, with a plastic film, and in a manner which varies considerably depending on the nature of the film, the drops of water from condensation can be much more prominent and dense; simple wiping removes the drops, but no film is formed and fogging is renewed more rapidly.
Various solutions have already been proposed to improve the behavior vis-a-vis the vapor of transparent surfaces in glass or plastic.
In general, it is known to deposit on the transparent surface products of the hydrophilic type which are capable of mixing with water when there is a risk of vapor by forming a fine and homogeneous film. For example, the deposit of a layer of glycol or glycerin compounds considerably improves the properties of the treated surfaces However, these water-soluble compounds are removed with the first wiping and must therefore be continually renewed, which is restrictive and costly.
Also known, for example from German Patent No. 1,928,409, are methods for treatment of transparent surfaces so as to provide them with good resistance to fogging, which consist of depositing polymer layers of the polyacrylate and/or polymethacrylate type with hydroxyl groups. These very hydrophilic layers absorb water without being dissolved therein. Due to the absorption of water, these films swell to a greater or lesser degree, soften and become more sensitive to abrasion which, in the long term, is harmful to the transparency of the treated surface.
The perfection of anti-fogging treatments by the deposit of anti-fog surface layers has therefore always confronted contradictory requirements: an improved anti-fogging action decreases the abrasion resistance, while increased abrasion resistance corresponds to a decrease in the anti-fogging effect. In effect, it has been sought to reinforce the resistance of the layers by introducing a hardener into their compositions. However, since said hardener generally reacts by polycondensation with the constituent polymers of the layer, it renders unavailable the hydroxyl type groups which provide the layer with the hydrophilic properties which promote the anti-fogging action.
In the case of glazings coated with an anti-lacerating and auto-cicatrizable plastic layer, such as are described for example in the above-cited French and European patent publications, the problem of fogging must be resolved by a treatment of the glazing which in no way alters the excellent anti-laceration and auto-cicatrization properties which are specific to them by means of the presence of the transparent polyurethane coating layer; the treatment must be effective over a very long period of time; it must not modify the nature of the polyurethane layer, which would further risk altering the properties which it confers to the glazing.
It has been proposed in European Patent Publication No. 0,133,111 to incorporate at least a tensio-active agent into the reticulated structure of the polyurethane layer. Reticulated is used to mean a structure comprising a certain number of bridging bonds between linear or branched chains. Due to the existence of a three-dimensional system, it seems that the tensio-active agents find anchoring points in the cross-linked structure and remain there durably fixed.
The tensio-active agent penetrates the layer while retaining the known hydrophilic properties of said tensio-active agent. Due to the reserve thus available, the anti-fogging effect persists for a much longer time. Nevertheless, this effect is decreased over time and the proposed solution is therefore not completely satisfactory.
The object of the invention is therefore a fog-resistant, auto-cicatrizable, anti-lacerating, transparent polyurethane layer, which can be used in the above-described safety glazings, as well as a method of manufacture of said layer.